This invention was made with Government support under contract. The government has certain rights in this invention.
1. Field of the Invention
This invention relates generally to spacecraft attitude determination and control systems, more particularly, to a method and apparatus for correcting star tracker high spatial frequency (HSF) error to improve-attitude determination performance in stellar inertial attitude determination systems.
2. Description of the Related Art
Satellite navigation systems typically include an attitude determination and control system. In such systems, the spacecraft inertial attitude is determined continuously in the spacecraft control processor (SCP) by numerically propagating the measured spacecraft inertial rates provided by three orthogonal gyros. Periodically, star data (star positions, star magnitudes, and their time tags) provided by the star trackers are then used to correct the attitude errors caused by gyro bias and gyro angle random walk during attitude propagation. The resulting attitude determination performance depends heavily on star tracker performance in terms of its measured star position accuracy.
Star trackers measure the positions of stars in the star tracker field-of-view (FOV). Several types of errors typically corrupt star tracker position measurements, thereby resulting in attitude determination errors. These star tracker errors can be generally attributed to temporal noise (that changes over time), high spatial frequency error that changes rapidly as stars move across the FOV, and low spatial frequency (LSF) error that changes slowly as stars move across FOV.
What is needed is a system and method for reducing spatial errors that degrade attitude determination and control systems. The present invention satisfies that need by post-processing the star data so that systematic HSF error can be removed without a need for real time control of star tracker boresight orientation.
To address the requirements described above, the present invention discloses a method and apparatus for post processing a star tracker measurement to remove a systematic error characterizable at least in part by a pixel phase. The method comprises the steps of computing the pixel phase along a first axis from a measured star position and a star tracker characteristic, computing a first axis error correction according to the computed pixel phase, and computing a compensated first axis star tracker measurement according to the measured star position and the first axis error correction. The apparatus comprises a first module, communicatively coupled to a star tracker, the first module configured to compute the pixel phase along a first axis from a measured star position and a star tracker characteristic a second module communicatively coupled to the first module, the second module configured to compute a first axis error correction according to the computed pixel phase, and a third module communicatively coupled to the second module, the third module configured to compute a compensated first axis star tracker measurement according to the measured star position and the first axis error correction.